Ayub Khan, the local police chief, said: "The locality has been a hub of militants. Locals set up a militia and expelled the militants from this area. This attack seems to be reaction to their expulsion."

He said the bomber drove a vehicle loaded with 250kg of high-intensity explosives onto the field, which lies in a congested neighbourhood, during the volleyball contest.

Some nearby houses collapsed, and "we fear that some 10 or so people might have been trapped in the rubble", he said.

In addition, a group of local tribal elders were holding a meeting at a mosque nearby. The mosque was damaged and some people there died, he said.

Drone raid

Earlier on Friday, missiles fired by a US drone killed at least three anti-government fighters in North Waziristan, security officials said.

The attack took place early morning in Ghundikala village near the town of Mir Ali.

"A US drone fired two missiles, targeting a vehicle and killing three militants," a senior security official in the area told the AFP news agency.

North Waziristan houses Taliban and al-Qaeda fighters as well as members of the Haqqani network, a powerful group known for staging attacks on foreign troops in Afghanistan.

Another US drone attack late on Thursday killed at least three fighters when two missiles hit their hideout.

The use of so-called Unmanned Aerial Vehicles (UAVs), which allow the military to operate in highly dangerous areas, is expected to grow in the coming years with the US defence department expected to buy 700 drones next year alone.

Karachi shutdown

Against this backdrop of continued violence, Karachi, the country's largest city, came to a virtual standstill on Friday.

Religious and political leaders had called for a general strike in protest against Monday's bombing that killed 44 people during a procession of Shia Muslims, and subsequent riots.

The city's major markets, stores and business centres were closed, along with financial institutions that had already planned to shut because of New Year's Day.

Public transportation was halted and petrol stations were shut down.

Rehman Malik, the Pakistani interior minister, said investigators were still determining if the attack was a suicide bombing.

He also questioned the claim of Asmatullah Shaheen, a purported Taliban spokesman, that the group was behind the attack.